The present invention relates to a thermal link device between the end of the cold finger of a cryogenic refrigerating machine and a load which is to be taken to a cryogenic temperature while it is in use.
The invention has a particularly important, although not exclusive, application when the refrigerating machine operates by using the Stirling cycle. The invention is nevertheless also suitable for use when said machine uses some other closed cycle or indeed an open cycle, e.g. the Joule-Thomson cycle.
Machines of the above kind deliver low temperature to the end, generally constituted by a thick cover, of a cold finger whose base is directly or indirectly in contact with an environment at a higher temperature. To reduce losses by conduction, a tube is used that has a very thin wall of material with low thermal conductivity, such as stainless steel or titanium. Since the tube is thin, it simultaneously presents very low mechanical strength and very low stiffness. Any force exerted on its end can consequently deform the cold finger, and that can have consequences that are particularly severe when the finger contains a moving element, as is the case in Stirling cycle machines.
Attempts have therefore been made to implement thermal link devices which simultaneously have low thermal resistance and apply only small forces to the end of the cold finger. In particular, thermal link devices have been made that are constituted by a braid of copper wires whose mass and stiffness are as small as possible. That solution is nevertheless not entirely satisfactory. A braid of low mass and stiffness has high thermal resistance. In order to assemble the braid to the cover of the cold finger, it is necessary to have direct access to the finger and to the load, and that is difficult to make compatible with achieving high performance thermal insulation. The fragility of the cold finger makes assembly difficult. In order for the braid to have the required flexibility, its length and volume must be large.
The use of a thermal braid suffers from an additional drawback when a single load is cooled by two machines, for the purpose of providing redundancy. If one of the machines is stopped, e.g. because of a breakdown, then the parasitic heat loss through the cold finger of that machine, which remains thermally linked to the load, is added to the power required by the load.
Also known, from U.S. Pat. No. 4,802,345, is a thermal link device between a cold finger and a load, the device being constituted by a narrow gap containing gases, at least one of which is incondensable at the operating temperature. The narrow size of the gap is essential and makes decoupling difficult.
Document U.S. Pat. No. 4,178,775 describes a cryostat for an infrared detector cooled by an open cycle refrigerator machine. Blotting paper retains liquefied gas close to the infrared detector. The blotting paper does not act as a pump, but only as a storage.